The present invention relates generally to electrical machines, and more particularly to a multi-rotor generator for use in aircraft.
Most modern aircraft are primarily powered by gas turbine engines. Gas turbine engines are Brayton cycle engines comprising a compressor, a combustor, and a turbine. The compressor pressurizes environmental air, typically via multiple stages of rotary blades and stationary vanes. The combustor injects fuel into the high-pressure air stream provided by the compressor, and combusts the resulting fuel-air mixture. The turbine extracts energy from the resulting high-pressure, high-temperature airflow. This extracted energy is used to drive the compressor, and to power other systems. Many aircraft gas turbine engines are connected to rotary generators via mechanical gearboxes to supply electrical power to aircraft systems. The power and propulsion demands of commercial aircraft continue to increase.
Most modern commercial aircraft use multi-stage gas turbine engines with separate high and low pressure spools. High pressure turbines operate at high speeds, extracting energy from gas leaving the combustor to drive high pressure compressor stages immediately upstream of the combustor via a high pressure shaft. Low pressure turbines operate at much lower speeds but considerably more variable speed range, and extract energy from gas leaving the high pressure turbine. Low pressure turbines are connected via low pressure shafts which drive low pressure compressors. Gearboxes for electrical generators are conventionally attached to the high pressure shaft, and extract energy from the high pressure spool. Gearboxes can add considerable weight and mechanical complexity to turbine/generator systems.
Many generator systems for aircraft use generation systems with three rotating machines on a common shaft: a main generator, an exciter, and the permanent magnet generator (PMG). The largest of these is the main generator, commonly of a wound rotor synchronous type. The rotor of a generator of this type has a field winding that is provided with DC current to create a magnetic field that spins in synchronism with the rotor. The exciter commonly consists of a stationary field winding with a rotating armature winding. The alternating current from the rotating armature winding is rectified to a direct current using a rotating rectifier and is fed to the wound rotor of the main generator. The current in the field winding of the exciter is controlled by a generator control unit (GCU) to provide the required output from the main generator. The GCU may be designed such that it can be powered by either the main generator or the permanent magnet generator (in the event that the main generator is not yet providing electricity). The permanent magnet generators comprise permanent magnet rotors with wound stators connected to the GCU.
Turbofans are often classified by bypass ratio, the ratio of bypass airflow (i.e. airflow through a fan which bypasses the compressor, combustor, and turbine) to compressor airflow. The trend in recent engines is for higher bypass ratios, with the result that less power is available from the high pressure spool for electrical power generation.